71
Origin and Occurrence
adjustments to refinery operations to handle these heavier crude oils to reduce the amount of coke
formed during processing and to balance the overall product slate.
3.3.3 nAturAl gAs
Natural gas is the gaseous mixture associated with petroleum reservoirs and is predominantly
methane, but does contain other combustible hydrocarbon compounds as well as nonhydrocarbon
compounds (Speight, 1993, 2007; Mokhatab et al., 2006). Natural gas has no distinct odor and the
main use is for fuel, but it can also be used to make chemicals and LPG.
The gas occurs in the porous rock of the Earth’s crust either alone or with accumulations of
petroleum (Figure 3.2). In the latter case, the gas forms the gas cap, which is the mass of gas trapped
between the liquid petroleum and the impervious cap rock of the petroleum reservoir. When the
pressure in the reservoir is sufficiently high, the natural gas may be dissolved in the petroleum and
is released upon penetration of the reservoir as a result of drilling operations.
There are several general definitions that have been applied to natural gas. Thus, lean gas is gas
in which methane is the major constituent. Wet gas contains considerable amounts of the higher
molecular weight hydrocarbons. Sour gas contains hydrogen sulfide whereas sweet gas contains
very little, if any, hydrogen sulfide. Residue gas is natural gas from which the higher molecular
weight hydrocarbons have been extracted and casing head gas is derived from petroleum but is
separated at the separation facility at the wellhead.
To further define the terms dry and wet in quantitative measures, the term dry natural gas indicates that there is less than 0.1 gal (1 gal, US, = 264.2 m 3 ) of gasoline vapor (higher molecular
weight paraffins) per 1000 ft 3 (1 ft 3 = 0.028 m 3 ). The term wet natural gas indicates that there are
such paraffins present in the gas, in fact more than 0.1 gal/1000 ft 3 .
Associated or dissolved natural gas occurs either as free gas or as gas in solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas whereas the gas that exists in
contact with the petroleum (gas cap) is associated gas.
Other components, such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), mercaptans (thiols;
R–SH), as well as trace amounts of other constituents, may also be present. Thus, there is no single
composition of components that might be termed typical natural gas. Methane and ethane constitute the bulk of the combustible components; carbon dioxide (CO 2 ) and nitrogen (N 2 ) are the major
noncombustible (inert) components.
Some natural gas wells also produce helium, which can occur in commercial quantities; nitrogen
and carbon dioxide are also found in some natural gases. Gas is usually separated at as high a pressure as possible, reducing compression costs when the gas is to be used for gas lift or delivered to a
pipeline. After gas removal, lighter hydrocarbons and hydrogen sulfide are removed as necessary to
obtain a crude oil of suitable vapor pressure for transport yet retaining most of the natural gasoline
constituents.
In addition to composition and thermal content (Btu/scf, Btu/ft 3 ), natural gas can also be characterized on the basis of the mode of the natural gas found in reservoirs where there is no or, at best
only minimal amounts of, crude oil.
Thus there is nonassociated natural gas, which is found in reservoirs in which there is no, or
at best only minimal amounts of, crude oil. Nonassociated gas is usually richer in methane but is
markedly leaner in terms of the higher molecular weight hydrocarbons and condensate.
Conversely, there is also associated natural gas (dissolved natural gas) that occurs either as free
gas or as gas in solution in the crude oil. Gas that occurs as a solution with the crude petroleum is
dissolved gas, whereas the gas that exists in contact with the crude petroleum (gas cap) is associated gas. Associated gas is usually leaner in methane than the nonassociated gas but is richer in the
higher molecular weight constituents.
Another product is gas condensate that contains relatively high amounts of the higher molecular
weight liquid hydrocarbons. These hydrocarbons may occur in the gas phase in the reservoir.
Origin and Occurrence
adjustments to refinery operations to handle these heavier crude oils to reduce the amount of coke
formed during processing and to balance the overall product slate.
3.3.3 nAturAl gAs
Natural gas is the gaseous mixture associated with petroleum reservoirs and is predominantly
methane, but does contain other combustible hydrocarbon compounds as well as nonhydrocarbon
compounds (Speight, 1993, 2007; Mokhatab et al., 2006). Natural gas has no distinct odor and the
main use is for fuel, but it can also be used to make chemicals and LPG.
The gas occurs in the porous rock of the Earth’s crust either alone or with accumulations of
petroleum (Figure 3.2). In the latter case, the gas forms the gas cap, which is the mass of gas trapped
between the liquid petroleum and the impervious cap rock of the petroleum reservoir. When the
pressure in the reservoir is sufficiently high, the natural gas may be dissolved in the petroleum and
is released upon penetration of the reservoir as a result of drilling operations.
There are several general definitions that have been applied to natural gas. Thus, lean gas is gas
in which methane is the major constituent. Wet gas contains considerable amounts of the higher
molecular weight hydrocarbons. Sour gas contains hydrogen sulfide whereas sweet gas contains
very little, if any, hydrogen sulfide. Residue gas is natural gas from which the higher molecular
weight hydrocarbons have been extracted and casing head gas is derived from petroleum but is
separated at the separation facility at the wellhead.
To further define the terms dry and wet in quantitative measures, the term dry natural gas indicates that there is less than 0.1 gal (1 gal, US, = 264.2 m 3 ) of gasoline vapor (higher molecular
weight paraffins) per 1000 ft 3 (1 ft 3 = 0.028 m 3 ). The term wet natural gas indicates that there are
such paraffins present in the gas, in fact more than 0.1 gal/1000 ft 3 .
Associated or dissolved natural gas occurs either as free gas or as gas in solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas whereas the gas that exists in
contact with the petroleum (gas cap) is associated gas.
Other components, such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), mercaptans (thiols;
R–SH), as well as trace amounts of other constituents, may also be present. Thus, there is no single
composition of components that might be termed typical natural gas. Methane and ethane constitute the bulk of the combustible components; carbon dioxide (CO 2 ) and nitrogen (N 2 ) are the major
noncombustible (inert) components.
Some natural gas wells also produce helium, which can occur in commercial quantities; nitrogen
and carbon dioxide are also found in some natural gases. Gas is usually separated at as high a pressure as possible, reducing compression costs when the gas is to be used for gas lift or delivered to a
pipeline. After gas removal, lighter hydrocarbons and hydrogen sulfide are removed as necessary to
obtain a crude oil of suitable vapor pressure for transport yet retaining most of the natural gasoline
constituents.
In addition to composition and thermal content (Btu/scf, Btu/ft 3 ), natural gas can also be characterized on the basis of the mode of the natural gas found in reservoirs where there is no or, at best
only minimal amounts of, crude oil.
Thus there is nonassociated natural gas, which is found in reservoirs in which there is no, or
at best only minimal amounts of, crude oil. Nonassociated gas is usually richer in methane but is
markedly leaner in terms of the higher molecular weight hydrocarbons and condensate.
Conversely, there is also associated natural gas (dissolved natural gas) that occurs either as free
gas or as gas in solution in the crude oil. Gas that occurs as a solution with the crude petroleum is
dissolved gas, whereas the gas that exists in contact with the crude petroleum (gas cap) is associated gas. Associated gas is usually leaner in methane than the nonassociated gas but is richer in the
higher molecular weight constituents.
Another product is gas condensate that contains relatively high amounts of the higher molecular
weight liquid hydrocarbons. These hydrocarbons may occur in the gas phase in the reservoir.